Index: src/wp-content/themes/twentyseventeen/style.css
===================================================================
--- src/wp-content/themes/twentyseventeen/style.css	(revision 38881)
+++ src/wp-content/themes/twentyseventeen/style.css	(working copy)
@@ -1041,6 +1041,14 @@
 	background: #bbb;
 }
 
+input[type="search"] {
+	-webkit-appearance: none;
+	-moz-appearance: none;
+	-webkit-border-radius: 2px;
+	border-radius: 2px;
+	width: 100%;
+}
+
 /*--------------------------------------------------------------
 # Formatting
 --------------------------------------------------------------*/